Tetrahydrobiopterin (BH4) is an enzyme cofactor for various aromatic amino acid hydroxylases, including phenylalanine, tyrosine and tryptophan hydroxylases, as well as being an important cofactor for other enzymes such as the nitric oxide synthases (inducible NOS (iNOS), endothelial NOS (eNOS), and neuronal NOS (nNOS)), and alkylglycerol monooxygenase. As such, BH4 is involved in regulating production of various neurotransmitters (e.g., serotonin and dopamine) and nitric oxide. Deficiencies in BH4 are associated with deficiencies in neurotransmitters including serotonin and dopamine. Reduced BH4 production is also associated with reduced pain sensitivity after injury.
The last step in the de novo pathway for BH4 biosynthesis is conversion of 6-pyruvoyltetrahydropterin to BH4 by the action of sepiapterin reductase. Sepiapterin reductase (SPR) is inhibited by the clinically approved drug sulfasalazine and other sulfa drugs, thereby interfering with the de novo biosynthesis of BH4.